In the manufacture of semiconductor devices, typically thousands of individual transistor devices are formed upon a single silicon substrate. These devices are interconnected to form complex circuits, also known as integrated circuits, as required for a particular circuit design. Because the transistors are formed within the same substrate, the transistors must be electrically isolated except as interconnected according to the circuit design. In the absence of electrical isolation, undesired electrical connections between the transistors would cause circuit shorts. Several methods exist for device isolation and vary with the type of device being manufactured. One device isolation method widely used in the manufacture of metal-oxide-semiconductor (MOS) and bipolar-complementary-MOS (BiCMOS) devices is known as local-oxidation-of-silicon (LOCOS).
In the LOCOS process, a silicon nitride mask is used to prevent oxidation of the active surface regions of a semiconductor substrate. A pad oxide layer is formed under the silicon nitride mask to relieve stress induced in the substrate during the LOCOS oxidation process. The LOCOS process offers high reliability and proven high volume manufacturing compatibility. However, a major problem with the LOCOS technique is the loss of active surface area caused by the inability to control the lateral growth of the electrical isolation regions.
The phenomenon known as lateral encroachment occurs when the final width of the isolation region is larger than the intended width, which is defined by patterning the silicon nitride mask. The amount of encroachment is thought to be related to the pad oxide thickness near the edge of the silicon nitride mask. Oxidation under the silicon nitride mask occurs when oxygen diffuses through the pad oxide layer, and reacts with the underlying silicon substrate.
The desire to suppress the lateral encroachment experienced by the LOCOS process has led to the development of techniques intended to retard the oxygen diffusion through the pad oxide layer. One approach is to reduce the thickness of the pad oxide layer to some minimal value, thus reducing the cross-sectional area available for oxygen diffusion. Prior to silicon nitride deposition, a layer of polysilicon is deposited over the thin pad oxide layer. The layer of polysilicon is used to relieve compressive stress in the substrate. This method, known as poly-buffered-LOCOS, or PBL, achieves a slight reduction in oxide encroachment, however the process is difficult to control and requires the removal of both silicon nitride and polysilicon following the oxidation process.
Other techniques have been developed, which include covering the edge of the pad oxide layer with a second layer of silicon nitride, or a silicon dioxide layer. By covering the edge of the pad oxide layer, the pad oxide layer is protected from exposure to oxygen during the oxidation process. Also, the additional dielectric layer provides mechanical resistance to the oxidation of the silicon underlying the pad oxide layer.
More recently, LOCOS improvement techniques have been developed which reduce lateral encroachment by forming a recessed region underneath the silicon nitride layer. The recessed region is then filled with polysilicon prior to carrying out the oxidation process to form the isolation regions. This method is known as polysilicon-encapsulated-local-oxidation (PELOX) and is described by Roth, et al. in U.S. Pat. No. 4,927,780, entitled "Encapsulation Method for Localized Oxidation of Silicon." Further improvements of the PELOX method are described by Vasquez, et al. in U.S. Pat. No. 5,175,123, entitled "High-Pressure Polysilicon Encapsulated Localized Oxidation of Silicon." Although, the PELOX method is effective at reducing lateral encroachment during a LOCOS process, further development is necessary to enhance the PELOX process for the formation of deep isolation regions in semiconductor substrates. Accordingly, further improvements will be realized by the present invention, as will be evident from the description which follows.